The eyes do not stay perfectly still during visual fixation. Fixational eye movements and saccadic intrusions continuously change the position of gaze. Here we focus on the most common type of saccadic intrusion: square-wave jerks (SWJs). SWJs are characterized by one small horizontal saccade that moves the eye away from the fixation target, followed by a corrective saccade towards the target shortly thereafter. SWJs are prevalent in some neurological diseases such as progressive supranuclear palsy (PSP). However, they are also common in normal subjects. We developed an objective algorithm to automatically identify SWJs in PSP patients and healthy volunteers, during visual fixation of a small target. Our results show that, whereas SWJs were common in both PSP patients and normals, SWJs in the PSP group had significantly higher rates and magnitudes, and were more markedly horizontal in direction. Using ROC (receiver operator characteristic) analyses we determined that the deviation from horizontal direction is the parameter that best distinguishes the PSP patient population from the population of healthy volunteers. The objective characterization of SWJs may provide a powerful tool in the differential diagnosis of oculomotor disease.

Supported by: the Barrow Neurological Foundation, the Arizona Biomedical Research Commission, NIH grant EY06717, the Office of Research and Development, Medical Research Service, Department of Veterans Affairs, the Evenor Armington Fund, and the OASI Institute for Research and Care (IRCCS) on Mental Retardation and Brain Aging, Troina, Italy.